The objective of this research is to determine the mechanism and control of the electrical coupling between the two major cell types in the midgut epithelium, goblet cells and columnar cells. The specific goals are to discover (1) whether the intercellular coupling is linked to potassium active transport, whether the coupling is linked to calcium active transport, (3) whether intercellular coupling can be correlated with kinetic determinations of the fraction of midgut cells involved in the potassium active transport pool, (4) whether changes in the potassium and calcium levels in the diet can be correlated with changes in intercellular coupling. Methods of answering these questions start with isolation of the insect midgut from a fifth-instar larva from a suitable lepidopteran species (usually Hyalophora cecropia (L.)). The isolated midgut is then mounted on a plastic chamber so that the surface of the epithelium is visible through a microscope. Three microelectrodes are placed in adjacent cells, one a goblet cell and the other a columnar cell. The double electrode cell is then tested for electrical coupling to the single electrode cell by passing a current pulse from the interior or the double electrode cell to the bathing solution. The fraction of current appearing in the adjacent single electrode cell measures the electrical coupling between the cells. Calcium and potassium activities inside the cell will be determined by ion-sensitive dyes and ion-selective electrodes, respectively. The transepithelial calcium and potassium active-transport will be measured by isotopically labelled fluxes to relate diet ion concentration, transport pool size and intercellular coupling.